1. Field of the Invention
This invention relates to a non-destructive apparatus and method for evaluating the properties of superconducting materials. Such structures of this type generally allow the superconducting material to be tested without having to destroy the material or throw the material away.
2. Description of the Related Art
It is known in prior superconducting material evaluation systems to use either destructive or non-destructive systems. In particular, a destructive system has been developed for both short sample tests and long sample tests. In the short sample tests, sections of superconducting material (here referred to as tape), usually, between 4 inches to 4 feet long are removed from the entire length of prepared superconducting tape and tested by conventional testing procedures using a background electromagnetic field with the tape being immersed in liquid helium. It is to be understood that while superconducting tape is discussed, superconducting wire could also be used. The current that is run through the short sample is varied to see at what value the short sample quenches, and the current is compared with the results of a graph much like the one in FIG. 1 where current in amps is plotted against electromagnetic field strength (B). If the field strength is known and the temperature value is known, then the operator can extrapolate from the graph as to whether the short sample quenched is in agreement with the current at which a good tape should quench, given the same field strength and temperature values. If the short sample test falls within the guidelines as set forth in FIG. 1 for a particular current, electro-magnetic field, and temperature, then it is assumed that the entire tape is good. If the short sample does not fall within the guidelines, then it is assumed that the entire tape is defective. While the short sample method is a general way to determine if the entire tape is good or defective, the method is destructive because the sample must first be cut down and then thrown away after the testing. Also, the short sample only tests a little part of the entire tape, and maybe the rest of the tape is not defective even if the sample proves to be defective. Therefore, a more advantageous system, then, would be presented if the test could be performed in a non-destructive manner while testing a substantially longer portion of the tape or the entire tape.
In order to test a substantially longer portion of tape, usually, between 1000 to 7000 feet, a long sample, destructive evaluation system was developed. In this case, a long sample of superconducting material, typically, one inch wide is cut into 7 strips with each strip being approximately 3 mm in width. The outer two strips along each edge of the material are then epoxy impregnated according to a conventional practice. The outer two strips were selected because it was determined by conventional statistical analysis that the edges of the material are the places where defects in the material are most likely to be located. Also, the long sample has to be impregnated so that the sample could withstand the electromagnetic forces experienced by it when the current was run through the long sample. In other words, the impregnation provided the needed support for the sample so that the sample would not mechanically fail due to the current passing through it. These impregnated coils were then tested by using the same test procedure used to test the short samples except that a background electromagnetic field is not necessary. If the long samples fell within the guidelines as shown in FIG. 1, it was assumed that the entire tape was good. If the long sample was bad, it was assumed that the entire tape was bad. This long sample test provided a more accurate test as to the quality of the entire tape but the long sample still had to be destroyed because it was impregnated. Consequently, a still more advantageous system, then, would be presented if the advantageous longer samples were used but without having to destroy the samples.
Non-destructive tests for evaluating the quality of superconducting materials have been developed but, only for use in a non-cryogenic atmosphere, however. Beyond the obvious test of visually inspecting the tape, a conventional eddy current test or an ultrasound test was performed on the sample. While these tests pointed out obvious flaws in the tape, they did not test the actual superconducting characteristics of the tape because the superconducting characteristics should be determined at cryogenic temperatures. Consequently, a further more advantageous system, then, would be presented if the advantageous longer samples were used but were tested in an atmosphere in which a more accurate determination as to the quality of the sample could be attained.
It is apparent from the above that there exists a need in the art for an evaluation system for superconducting materials which determines the superconducting quality of the material, and which at least is capable of evaluating samples of a relatively long length, but which at the same time performs the evaluation in a non-destructive manner in an atmosphere that produces the most accurate information. It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.